Mobile telephony systems in which user equipment such as mobile handsets communicate via wireless links to a network of base stations connected to a telecommunications network have undergone rapid development through a number of generations. The initial deployment of systems using analogue modulation has been superseded by second generation digital systems, which are themselves currently being superseded by third generation digital systems such as UMTS and CDMA. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposal by the Third Generation Partnership Project of the so-called Long Term Evolution system, often simply called LTE, which offers potentially greater capacity still, by the use of wider frequency bands, spectrally efficient modulation techniques and potentially also the exploitation of spatially diverse propagation paths to increase capacity (Multiple In Multiple Out).
Distinct from mobile telephony systems, wireless access systems have also undergone development, initially aimed at providing the “last mile” (or thereabouts) connection between user equipment at a subscriber's premises and the public switched telephone network (PSTN). Such user equipment is typically a terminal to which a telephone or computer is connected, and with early systems there was no provision for mobility or roaming of the user equipment between base stations. However, the WiMax standard (IEEE 802.16) has provided a means for such terminals to connect to the PSTN via high data rate wireless access systems.
Whilst WiMax and LTE have evolved via different routes, both can be characterised as high capacity wireless data systems that serve a similar purpose, typically using similar technology, and in addition both are deployed in a cellular layout as cellular wireless systems. Typically such cellular wireless systems comprise user equipment such as mobile telephony handsets or wireless terminals, a number of base stations, each potentially communicating over what are termed access links with many user equipments located in a coverage area known as a cell, and a two way connection, known as backhaul, between each base station and a telecommunications network such as the PSTN.
FIG. 1 shows a conventional wireless cellular network; in this example, the access links of base stations 2a . . . 2g are arranged in a so called “n=3” frequency reuse pattern, that is to say that the available wireless frequency spectrum is divided into three sub-bands f1, f2 and f3, in which n signifies the number of sub-bands. The area of coverage of each base station is divided into three sectors by the use of directional antennas, and each of the sectors operates in a different frequency sub-band. In the example of FIG. 1, the sectors indicated by reference numerals 1a, 1b and 1c associated with the base station indicated by reference numeral 2a operate in frequency sub-bands f1, f2 and f3 respectively. It can be seen that the frequency re-use pattern shown in FIG. 1 can be repeated without adjacent sectors operating at the same frequency, thereby minimising interference between adjacent sectors. Sub-bands need not necessarily consist of contiguous blocks of frequencies, indeed there may be some advantage in interleaving the frequencies in order to distribute the effect of frequency selective fades. A frequency selective fade is a reduction in signal power due to destructive interference between multipath components. In cellular systems employing orthogonal frequency division multiplexing (OFDM) such as, for example WiMax or LTE, a sub-band will typically comprise a non-contiguous groups of sub-carriers; However, for clarity the sub-bands are often illustrated as being separate contiguous blocks, in which the numerical designation of a frequency has an arbitrary relationship with the actual frequency at the physical layer.
FIG. 1 is a schematic diagram in which sectors 1a, 1b, 1c are shown as hexagonal areas; in practice, geographical constraints and propagation conditions will cause the area of coverage of each sector to be irregular in shape and the areas of sectors to be unequal and the spacing of base stations will be determined by available sites and will not necessarily correspond to the idealised situation shown in FIG. 1.
There may be gaps in the area of coverage of a cellular system due to shadowing by the terrain or by interference between signals transmitted by base stations. Conventionally these gaps may be countered by the use of repeater stations that receive signals from a base station and re-transmit them into an area where coverage is poor. However, a repeater station that simply retransmits all signals received within a band may cause interference that reduces coverage in other areas. The interference may be reduced by using a relay station instead of a repeater station; a relay station selects which signals to retransmit, typically transmitting to terminals within the area of poor coverage.
Typically, a relay station is a small low power base station with an omni-directional antenna, in contrast to a conventional base station, which typically operates with a higher transmit power than a relay station, and typically employs directional antennas that are mounted on a tower to give an extensive area of coverage. The radio resource of the cellular wireless system may be used to relay backhaul traffic between a relay station and a conventional base station.
FIG. 2 shows a conventional relay station operating within a cellular wireless network; the operation may for example be in accordance with IEEE 802.16j. A user equipment 12b is in communication with a relay station 10. As the relay station 10 is not provided with a backhaul link separate from the cellular wireless resource, the relay station is allocated radio resource timeslots for use relaying backhaul data to and from the adjacent base station 2a which is itself connected by microwave link to a microwave station 6 and thence to a telecommunications network 8 such as the public switched telephone network. A user equipment 12a is shown in direct communication with the base station 2a. 
The relay station 10 may be deployed in an area partially obscured from base stations by a geographical feature such as a hill or another obstruction such as buildings, or within a building to give coverage to parts of the building that experience a poor link or no coverage from a base station. The relay station 10 is positioned such that it can communicate with a base station, and also give coverage to an obscured area. Typically, the relay station is required to give coverage to a smaller area than that covered by a base station sector. Conventionally, relay stations are used to cover a small proportion of the area of wireless coverage of the cellular wireless system, and the coverage areas of relay stations rarely overlap each other. In such a conventional low density deployment of relay stations, the allocation of operating frequencies to relay stations for communication with user equipment may be carried out in an ad hoc manner; it may be acceptable to re-use the frequency sub-band allocated to the base station sector within which the relay station is deployed, if the area of overlap is small between the coverage of the relay station and that of the base station. Alternatively, a different sub-band may be allocated to the relay station from that allocated to the base station sector within which the relay station is deployed. Provided that the area of coverage of the relay station is small, the potential for interference with signals in other base station sectors and with signals from other relay stations may not be an issue.
However, there is potentially an advantageous use of relay stations for the purpose of increasing the capacity of a wireless cellular network in general, not limited to situations in which parts of the target areas of coverage are obscured from base stations. Such a general use of relay stations could potentially involve a high density deployment of relay stations within a base station sector, such that the coverage areas of relay stations may overlap with each other and also overlap substantially with the areas of coverage of base station sectors. The potential advantage of such a deployment is that relay stations would provide local areas of signal reception in which the carrier to interference ratio is improved over that provided by the base stations alone. However, it may be problematic to allocate frequency sub-bands to relay stations deployed within a conventional cellular wireless network employing n=3 frequency re-use in a way that does not result in interference.
FIG. 3 illustrates the potential problems of deployment of relay stations 10a . . . 10c within a cellular wireless system using an n=3 frequency re-use scheme, showing the area of coverage of two base stations 2a, 2b. Three relay stations 10a . . . 10c are deployed within the area of coverage of the base stations 2a, 2b, and three user equipments 12a . . . 12c are shown. A given user equipment 12a can receive signals from both a base station 2a and a relay station 10a. The user equipment 12a will hand over to use whichever of the base station 2a and relay station 10a provides the highest quality signal, which quality may be expressed in terms of carrier to interference ratio. The aim of the hand over process is to increase the average carrier to interference ratio available within the area of coverage of the wireless cellular system and hence increase the traffic capacity, since the traffic capacity is related to the carrier to interference ratio.
The allocation of frequency sub-bands to the system as illustrated by FIG. 3 is problematic, taking, for example, the case of the relay station indicated by reference numeral 10a. If this relay station 10a were to be operated at frequency sub-band f1 as used by base station sector 1a, there is potential for interference between signals transmitted from relay station 10a and those transmitted by the base station 2a. If the relay station 10a were to be operated at frequency sub-band f2, there is potential for interference between signals transmitted from relay station 10a and those transmitted by the base station 2b in the sector 1e in communication with user equipment 12b. In the case that the relay station 10a were to be operated at frequency sub-band f3, there is potential for interference between signals transmitted from relay station 10a and those transmitted by the base station 2a in the sector 1c in communication with user equipment 12c. 
FIG. 4 shows a conventional time frame structure allocating timeslots alternately to access 14a . . . 14d and to backhaul, also referred to as “relay” 16a . . . 16c between a relay station 10 and an associated base station 2, in a system such as that illustrated in FIG. 2.
FIG. 5 shows an example of the conventional allocation of radio resource within each of the access time slots 14a . . . 14d of the frame structure of FIG. 4. In a system not employing relays, the relay timeslots may be absent, so that the access timeslots are contiguous in time. The radio resource is split in frequency into three sub-bands f1, f2, f3 for use in a n=3 re-use pattern such as that illustrated in FIG. 1. It can be seen that each frequency sub-band is divided in the time dimension into control timeslots 18a . . . 18c and payload 20a . . . 20c timeslots, and that the control timeslots 18a . . . 18c for the frequency sub-bands f1, f2 and f3 coincide with one another in time. This coinciding in time occurs since a user equipment receiver 12a, 12b is synchronised to the radio resource frame structure and the receiver 12a, 12b is pre-programmed in accordance with the relevant cellular wireless standard such as, for example, the WiMax or LTE standard, to expect to receive control data at the same time in each sub-band. An example of the data that would form part of the control timeslot 18a . . . 18c is the frame control header (FCH) in the 802.16 WiMax system. Similarly, in the case of LTE systems, there are control timeslots which may be located at various positions within the data frame; for example, control timeslots may be located at the beginning, middle and end of a frame. In general, control traffic may, for example, indicate the size of a frame and its start and stop addresses.
In order to receive the payload part 20a . . . 20c of a frame, it is necessary to receive the respective control timeslot 18a . . . 18c associated with the frame. It is thus particularly important that the control timeslots 18a . . . 18c be protected from interference. In the n=3 frequency reuse scheme illustrated by FIG. 1, interference between control timeslots of signals transmitted by adjacent sectors of any given base station 2a is inherently minimised since, as has already been mentioned, adjacent sectors operate at different frequency sub-bands.
Typically, the information carried by the control timeslots will vary between base stations and between base station sectors. Therefore, techniques that mitigate the effects of interference between base stations and between base station sectors by the intelligent combination of potentially interfering signals that carry the same information are not generally applicable for use with control timeslots. For example, soft handover and best server selection methods are generally not applicable for use with the control timeslot as they would impose the limitation that the information content of potentially interfering signals would be the same.
While it may be possible to control the allocation of radio resource within the payload part of the frame 20a, 20b, 20c to avoid interference between signals from the base station 2 and the relay station 10, it is typically not possible to re-allocate the radio resource used for control data 18a, 18b, 18c, since this is typically defined within the relevant cellular wireless standard to occur at pre-defined positions within the frame structure. User equipment operating to the relevant standard is thus pre-programmed to expect control data at the pre-defined positions within the frame structure. Therefore, if the same sub-band is allocated to the relay station 10 as to the base station 2, there is the potential for interference to occur between control data transmitted from the base station 2 and control data transmitted from the relay station 10.
In practice, relay stations are of most value when placed at the extremes of coverage of a base station sector, since it is here that augmentation of coverage are most likely to be required, but it is also in this situation that interference is most likely to be caused. In addition, interference may be experienced between transmissions from adjacent relay stations which may be operating in the same sub-band.
The use of a relay station within the area of coverage of a conventional cellular wireless network using n=3 frequency re-use thus can potentially cause interference with signals transmitted from base stations and with signals transmitted from neighbouring relay stations.
It is an object of the present invention to provide methods and apparatus which addresses these disadvantages.